1. Field of the Invention
One disclosed aspect of the embodiments relates to a photoelectric conversion device, a method for driving the photoelectric conversion device, and an image pickup system.
2. Description of the Related Art
General image pickup apparatuses such as digital still cameras and digital camcorders capture still images along with moving images.
Japanese Patent Laid-Open No. 2010-4175 discloses an image pickup apparatus having a mode for capturing still images along with moving images. In this mode, pixels in all rows in an imaging plane simultaneously perform accumulation operations. In the accumulation operations, electric charges generated in a certain period of time are accumulated in photoelectric conversion units of the pixels.
In a first frame, electric charges accumulated in the photoelectric conversion units are simultaneously transferred to charge holding units included in the individual pixels. In the first frame, assuming that n is an integer equal to or larger than 0, signals are read only from pixels in (5n+1)th row and (5n+2)th row. Thereafter, in a second frame following the first frame, the accumulation operations are simultaneously performed in all the pixels again. However, only electric charges accumulated in the photoelectric conversion units of the pixels in the (5n+1)th row are transferred from the photoelectric conversion units to the charge holding units. Thereafter, signals corresponding to the electric charges transferred to the charge holding units in the second frame are read from the pixels in the (5n+1)th row, and in addition, signals corresponding to the electric charges transferred to the charge holding units in the first frame are read from the pixels in (5n+3)th row. After a third frame onwards, operations the same as that of the second frame are performed. In this case, instead of the pixels in the (5n+3)th row, signals corresponding to the electric charges transferred to the charge holding units in the first frame are successively read from pixels in a (5n+4)th row and a 5n-th row.
Specifically, signals corresponding to the electric charges accumulated by the accumulation operations are read from the pixels in the (5n+1)th row for individual frames whereas signals are read from the pixels in the (5n+2)th, (5n+3)th, (5n+4)th, and 5n-th rows every four frames. In other words, four frames are required for reading signals corresponding to the accumulation operations in the first frame from all the pixels. A still image is configured using the signals corresponding to the accumulation operations in the first frame obtained as described above. As a result, a still image in which accumulation timings are highly synchronized may be obtained, and a moving image read in ⅕ density of the still image is updated at a speed four times higher than a speed of update of the still image.
In the technique disclosed in Japanese Patent Laid-Open No. 2010-4175, the pixels in (5n+3)th to 5n-th rows are not reset in the second to fourth frames. Therefore, in a period of time after the electric charges are transferred to the charge holding units of the pixels in the first frame and before signals are read in the second to fourth frames, if electric charges larger than saturation charge amounts of the photoelectric conversion units are generated in the pixels in the (5n+3)th to 5n-th rows, the electric charges may flow in the charge holding units included in the pixels in the (5n+3)th to 5n-th rows and adjacent pixels. When blooming occurs due to different accumulation timings or leakage of the charges generated in the pixels, quality of a resultant image is deteriorated.